In general, ink-jet inks are either dye- or pigment-based inks. Both are typically prepared in an ink vehicle that contains the dye and/or the pigment. Dye-based ink-jet inks generally use a liquid colorant that is usually water-based to turn the media a specific color. Conversely, pigmented inks typically use a solid or dispersed colorant to achieve color.
Color characteristics of dye-containing inks play an important role in the quality of the printed ink-jet image. Perceived color quality can be characterized using any one of several color space systems, such as CIELAB or Munsell, as is well known in the art. With respect to Munsell color space, a given color is defined using three terms, namely Hue, Value, and Chroma. With respect to CIELAB color space, a color is defined using three terms L*, a*, and b*. With this system, L* defines the lightness of a color, and it ranges from 0 to 100 (with 100 being white). Additionally, the terms a* and b*, together, define the hue, where a* ranges from a negative number (green) to a positive number (red), and b* ranges from a negative number (blue) to a positive number (yellow). Alternate terms such as h° (hue angle) and C* (chroma) can be used to further describe a given color, as is known to those skilled in the art.
There is a need to improve permanence of ink-jet produced photographs, while preserving their color vividness. There are several failure mechanisms by which ink-jet produced images can undesirably change color over time. Photographs can fade under the action of light or an atmospheric gas, or as a result of trace impurities in the air. Another failure mechanism is due to humidity causing humid color change. Prior solutions have used multiple dyes in a single ink wherein one provides good lightfastness and the other provides good chroma. Such systems in the prior art have lead to images having bright chroma and mediocre to good lightfastness on swellable photo media. Alternative, single bright dyes have been used that provide good humidfastness and airfastness, but relatively poor lightfastness, or vice versa.
Accordingly, investigations continue into developing ink formulations that have improved properties and that do not improve one property at the significant expense of others. Many challenges still remain to further improve the image quality and lightfastness of ink-jet prints without sacrificing pen performance and reliability.